A method of this kind is known from EP 0 563 575 A2, for example. In this method, a first tube is extruded into a mould tunnel comprising at least one row of moulds guided on a path. The first tube is given a corrugated shape in at least one first section and expanded into a connecting sleeve in at least one second section. A second tube is extruded into the first tube and pressed against the corrugation troughs of the first tube, thereby forming a compound tube comprising an outside tube and an inside tube welded to it. While the first tube is being given a corrugated shape and the second tube extruded into the first, the space between the two tubes is subjected to a pressure p1 that is above atmospheric pressure. Pressure p1 is selected such that, after cooling of the tubes that are welded together at the corrugation troughs, the inside tube does not bulge inwards or outwards between these points. After cooling of the tubes, the pressure should return to precisely atmospheric pressure.
The first tube is given a corrugated shape by externally applying a pressure that is below atmospheric pressure. Pressure p1 supports the corrugated shaping of the first tube, but is only relatively slightly above atmospheric pressure as a rule, in order to prevent outward bulging of the inside tube during cooling of the tubes, as described above.
According to EP 0 563 575 A2, the first tube is expanded into a connecting sleeve in the second sections by externally applying a partial vacuum. After expansion of the first tube to form a connecting sleeve, the space between the two tubes is vented to atmospheric pressure.
During extrusion of the second tube into the first tube, which has been expanded into a connecting sleeve, the second tube is internally subjected to pressure p3 above atmospheric pressure and pressed against the first tube. This ensures that full-surface welding of the two tubes is achieved in the region of the connecting sleeve.
After the connecting sleeve has been formed by both tubes and the second tube is again extruded against the corrugation troughs of the first tube in another first section of the same, pressure p1 is again applied to the space between the two tubes.
The external application of a partial vacuum on the first tube for the purpose of forming a connecting sleeve according to EP 0 563 575 A2 is difficult, because the space between the first tube and the associated section of the mould tunnel must be effectively sealed against the ingress of outside air. If sealing of this kind does not exist or is incomplete, the first tube is not expanded precisely in the associated section of the connecting sleeve. The required seal makes it necessary to take complex technical measures.